Signal discriminating system



May 23, 1944.

`GHODcsEs S IGNAL DI S GRIMINATING SYSTEM Fled'Aug. 28, 1940 -46' fr' X.. Z7

/N Vt' N TOR. Gro/Paf #026215.

Patented May 23, 1944 SIGNAL DISCRDVIINATING SYSTEM George Hodges, MeadowT Way, ickniaswoth, England Application August 28, 1940, Serial N0l1354,619 Y' In Great Britain August'ZS, 1939 1 claim. (c1. 25m-20N This" invention relates to Wireless signalling apparatus or other apparatus employing radio frequency methods and more particularly to the circuit arrangement'of a wireless receiving apparatus.

The object of the invention is to produce a circuit' arrangement for use with Wireless receiving apparatus which willv effect discrimination betweenwanted andi` unwanted signals, or between radierA wa'ves arriving by' more than one path from atran'smitterQ-or between a carrier wave and its sidebands.'

The' application' of the invention may be considered asfalling'principally into four categories:

1. As a means to effect the elimination or reduction of unwanted signals or interference in a wirele'ssf receiver".

A2.*As` a meansto elect a reduction of fading effects of received signals.

3. As a tuning circuit which provides a resonance'curvewhich isv iiat over a band of frequencies'orwhich may rise on each side of acarrier frequency;

According= to the present invention I provide ina' wirelesssignalling` apparatus or other apparatusemploying radio frequency methods, means for applying incoming signal potentials to an oscillatory circuit comprising aninductance, a resistance and a condenser in series, the said circuit being' associated with an impedance, one

side of theY impedance being connected to the junction" of the inductance and the resistance andthefother side of theimpedance being connecte'd` to earth; the circuit arrangement being such that bythe phase and amplitude matching adjustments provided, unwanted signal potentialsde'velopedv across the resistance and the impedance may beibalanced out so that they do not-reach' an output circuit, associated with the saidoscillatory circuit and impedance, at the same" time" permitting the Wanted signal potentials to reach the" said output circuit.

Inorder thatthe said invention may be clearly' unde'rstovfidI and readilyr carried into eie'ct the same will new beL described by wayk of eirample Withierence to' the"v accompanying drawing, in which? Figu'rci isc' circuit diagram showing the basic principles of the invention.

Figure 2'isay circuit-,diagram showing a modiiication'cf7 the circuit `shown in Figure 1.

Figure` is AaV circuit diagram showing a practic'al application of the circuit shown in Figurel.

Fig; 3a isc circuit diagram cf a circuit simmer to that illustrated' in Fig'. 3` but wherein the resistance is variable.

Fig. 3b is ai circuit diagram of a circuit similar to that'illustrated in Fig. 3 but wherein a, variable i condenser is applied in multiple to the resistance.

Fig. 3c is a circuit diagram of a circuit similar to thatillustrated in Fig. 3 but where the input and output' connections are illustrated in reversed relationship.

Figureiiis aV circuit diagram showing a modiiication of the invention in which the inductance is in the form of aloop aerial.

Fig. 5 is a circuit diagram similar to Fig. 4 but ofa modification wherein the impedance is in aloop circuit.

Figure 6 is' a circuit diagram showing a modiiication of the invention in which amplitude matching is achieved by thermionic Valves and Figure 7 is a circuit diagram showing a modicaticn of the invention in which the signal potentials are fed to two'ampliiiers.

Figure 8 is a circuit diagram showing a further modification ofthe invention in whichamplitude matching is achivedby thermionic valves feeding the signal potentials to two ampliers.

Figures'l" and2 illustrate the basic principles of the circuit arrangements; In Figure 1 an oscillatory circuit comprising an inductance L and aV condenser C has' connected in series a resistance R, the; circuit being earthed through an impedance Z. It is necessary that' the resistance should be connectedin the condenser side of the oscillatory circuit,` that isu to say between the condenser and'the impedance, the inductance be- 'ingv connected at one end to' the junction between the resistance and the" impedance and at the the'r end toA the side of the condenser opposite to the side connected to the resistance. The impedance" may be in' the form of a resistance or a capacity or an inductance or other suitable" combination of resistance, capacity and inductancel which produces an impedance of the required value between the oscillatory circuit and earth.` The output potential with respect to earth is taken from a tapping on the resistance of high value relative to the resistance it is advantageous to include a Xed condenser between the point X and the succeeding apparatus, the capacity of this condenser being kept as small as possible consistent with the requirement of conveying an adequate proportion of the wanted signal potentials to the succeeding apparatus.

In the basic circuit shown in Figure 1 the input` arrangements are omitted for simplicity. Signal potentials may reach the oscillatory circuit by the normal arrangements of coupling or connection to an aerial, with or without intermediate valve amplifying stages, or alternatively the inductance of the oscillatory circuit`A may be arranged as a loop aerial. On the output side the potential developed across the resistancev may be arranged to oppose the potential developed across the impedance-and provided the potentials are suitably matched in phase and amplitude unwanted signal potentials will not appear betweenthe pointX and earth.- l Y Y Phasing is accomplished by tuning the oscillatory circuit so that the potentials, developed acrossthe resistanceA and the impedance, ,are in phase or phase opposition, thel necessary condi- 1 tion, depending upon the form Vof amplitude matching adopted. Tuning of` the oscillatory circuit is performed in the normal manner by adjusting the inductance or .Capacity` or both. The` correct setting of the tuning controls to achieve phase matching across the resistance and the impedance is not4 necessarily exactlyl the saineas that required for resonance, though in operation the circuit is rst tuned roughly to the frequency of the signals to be dealt` with,a fine.

adjustment on either sidev of the resonant frequency then being made if necessary to achieve accurate phasematching. The values of the `.inductance and capacity of the oscillatory circuit may be chosen so ,that tuning is accomplished either at the fundamentalfrequency ofthe circuit or la harmonicfrequency. y

Aerials are of 4many kinds including a loop, a vertical conductor, a horizontal Aconductor and an umbrella. The theory of the phasing operaplitude is also set up across R by the vertical effect, the two potentials being out of phase as indicated in the preceding paragraph. Since they are of equal amplitude, the resulting potential will be a mixture of the two phases, i. e. the phase will be half way between the actual loop and vertical phases. But this new phase angle will apply only asvlong as the amplitudes are equal. If the loop aerial, without being rotated, picksup signals from a different direction, the relative amplitudes of the loop and vertical effects across R are changed for this second signal due to the directional effect. Although the potentials have the same phase difference as before, the fact that they have diierent amplitudes means that for this second signal the 'resulting phase of Ythe potential across R is not half Way between the loop and vertical phases, but will shift towards one or the other. Changes in the relative amplitudes of the loop and vertical eifects may be brought ,about not only by changes in the direction of, incoming signals. Any alteration of the tuningv of the loop circuit will cause a similar variation of the amplitude of the loop eifect across R, which, mixedV with the vertical effect across R, causes ,a phase shift with respect to the potential across A. Arn- 1 pltude and phase Vchanges across theimpedance tion may be explained by first considering a case where the` inductance of the voscillatory circuit is arranged as a loop aerial. Signal potentials are developed across the resistance R due to the loop aerial effect. `Signal potentials are developed across the impedance Z due to thevertical aerial Aeffect of an earthed loop. But it must be recognized that some potential due to vertical aerial effect will also be developed across the resistance R. The important point to be vobserved, therefore, vis that whilst the potential across Z is largely dueto vertical aerial eifect, the potential across R is a mixture of vertical andloop effects. in a vertical aerial is in phase with the flux in space and that in a loop circuit'the voltage is 90 degrees out of phase with the ux in space and that this phase angle is independent of the direction of the incoming waves relative to the loop.

The aforesaid terms loop and vertical effects are used to designate the eifectsobtained from a loopaerial and a vertical aerial, even though a portion of the former serves as the latter, as herein illustrated. Portions of the circuit including same are termed loop aerial and vertical aerial. Y

I take as anexample a case in which a potenthe loopeifect and a potential of the same am- Ilt is known that thevoltage.

Z should also be considered. Whilst the potential developed across Z islargely due to verticalaerial effect, the loop and Vertical aerial circuits cannot be regarded as being independent of each other due to the fact that the impedance is connected at a point which does not `earth the loop symmetrically and, accordingly, the amplitude and phase across Z is also modied by variations in the loop circuit. The total eifect of-all these possible phase changes is` sufficient toenable the tuning control ,to bring, the potentials across R and Z into phase; y y

A second example maybe considered in Vwhich the inductance is not a loop aerial, but thetuning coil of the circuit LCR, input potentials from a separate aerial or other source being applied 'betweenI a point on. the inductance, remote from earth, and earth. It is convenient toapproach the theory ofthis form of the` circuit by'analogy with the earth looppreviously described. Potentials developed in the circuit LCR will be referred to as being due tothe closed circuit effect,

potentials developed across the impedance Z.

difference between the potentials in the closedv circuit and the vertical circ-uit exists andas the tuning of the circuit is varied there isa variation in the mixtureofthe closed circuit and vertical elfects vacross R, vthe two potentials of different amplitude andphase producing a` new resulting phase as ini the example previously described. Theeifects are `closely, analogous to those lof the loop form of lthe apparatus, provided it is recognized'thatv in this second example there is no question of variations due to'directional effect, the tuning operation being employed` to compensate for phase changes due to differences of signal frequency.

An important eifectfshould be noted in con,- nection with this second example.V When the tuning of the` circuit LCR is adjusted* for phase matching the potential across R is in phase op position to the potential across Z, enabling Aamplitude balance to be obtained at the. output point X when `suitable values are chosen-for R'- and Z. The eiecti also exists in' the loop for-1n ofr the apparatus and although it contrib'ut'es to'r the phasechanges it is masked by the large variations which occur in the relative amplitudes ofthe loop-and vertical effects.

necessary withA the loop form of the apparatus when vphaseV opposition across .Brand-Z is desired.` A third example may be consideredinwhicn the. inductance is thetuning coil of the circuitW LCR, signals being applied to the circuit by inductivecoupling. with an` aerial or other source; It is. recognized vthat if the coupling' were purely inductive the theoreticalv conditions for phasel matching would not be fulfilled. In-A practice,- however, a stray capacity between the'iaerialcircuitand circuit LCR exists and it may be` encouraged by design tothe extent required. ,The i withthe inductance through an impedance 2.,.

the side ofv the` condenser connected to the impedance being earthed. Shunted-across the.: inductance of the circuit is afurther condenser- CI which may be 0f xed or variable value. ThisV condenser is arranged in series with a resistance Pt.` They circuit` of Figure 2 is similar to` that shown in Figure 1 except for the fact that an additional `condenser is connected between the end of the inductance, remotelfromearth, and

earth. 'Ijhe two circuits described operate on the same principle and for simplicityr the following description will relate to the firstmen-` tioned `circuit and its developmentsit being. understood that themodication illustrated in Figure 2 may be applied to various forms of thecircuit.

Accordingly,it will i be shown later that a` switching arrangementv Lifntionshowni signal potentials from an aerial or Ak practical applicationof the circuit is illuslatory circuit as previously described.` To matchthe amplitude of the two potentials thus opposed, a proportion of the potential developed across R' is tapped oif by the sliding contact shown. The impedance Z is adjusted to a pre-set value so that at some point on the resistance R thet signal potential between the tapping point and" the impedance is exactly equal'to the signal-l po'- tential across the impedance and consequently the unwanted signal potentials will not be passedl on to a receiver connected between theV point X and earth'.

InA this form' of' the' circuit discrimination" between"wanted and unwanted' signals dependsupon the 'difference in their'frequencies since' any difference in frequency will upset the phase" and vamplitude of the potentials developed across' the resistance and thehimpedance: Ifthe'circuit is" adjusted for balanceat thefrequency of' the" unwantedsignals; some-proportion' of'th'e wanted `ance as indicatede :fare reversible'.

signal; potentials atanoth'er' frequency will be passed through-to thef receiver; Whenthe unwant'e'dpotentialsy are due tol atmospherics or other electricaldistnrbances' having-V a'nindenite frequency characteristic', the' oscillatoryv circuit may be to a frequency' don'sideiably diierent from that of the wanted-signals. Some degre'eot` balance may? be aohiev'e'd forf the' interferencev whilst the wanted signals' force themselves throughto the-'output point.

Inlchoosingsuitable values" forthe` resistance and-'- tlfief impedance, v considerationmust be given to the particular form ofthe circuit used and the waveband over' whicliiti is designed to operate. In'` tlie' circuit shown in l'ligure13 a value of 100 or 200- ohr'ns' for' the;r resistance is satisfactory when thecirctisifoperating on the medium wave broadcastingrv band;` andv the' val-ue' `of the irnpedance is then'closensb thatthe potential de- .velo'pedl acrossv tlieimpedance' canr bemade equal to'- the potential developed acrossv the resistance. Higher or lower values for thez resistance and the impedance may be adopted provided their relative values* are' such as"to'give' amplitude bal- In the caseof short wave signals itis? preferable'- to use lower values so thatl the resistance' shall not damp' the' circuit unduly.

InFigurer-3iarnplitude'matching is provided by .-valrying-tlievallie-of tlie impedance Z and by ad'- ing-varies W t`o" the output point' X The value -of\ the differential condenser should be as small aS-practicable to-avoidundue capacity in parallelwith R. Alternative1 input-arrangements may be adopted: Instea'dof tle direct; aerial connecother source s'uch' as a'valve amplier may be applied to thefci'rcuit by norrnal directconnection or' inductiveor capacitive coupling arrangements. The input-,fand output vconnections of the circuitV Signals-from an aerial or other source maybe' applied be't'w'eer'i the point X and' earth,l andthe outf'put; seeFi'g-L 3c, may be taken front the end of'theinduct'ance L, remote from earth, andearth. Balance is achievedbyfmatch- ".ing thephaseand amplitude of the potentials due to-coupling by the' resistance R- and the impedanceZ, the basic principlesof lthe operation remaining' the sam'e as previouslyl described'.

In Figure 4 the iridiictanc'el L takes the form of a loop" aerial` (with or`without`a loading in'- ductance Vfor'tuning purposes)A and with this loopaerial arrangement the input connections previouslyv referred to are unnecessary. Under these conditions thepoten'tials developed across the resistance R and the impedance Z-may' either assist or oppose eachotheri-dependin'g upon the direction from whiclrthe' signals reach-the loop` aerial.. The'` possible Variability in the sign of the potentials maybe allowed for by reversing 'theconnection's'to the=1oop inducta'nce by means of al suitable switch' S; Unwanted signals may be balanced4 out by fulllin'g thepreviouslydescribed! requirements for niatclngf the phase and amplitude of the potentials developed'ac'roszsv for rotatingthe loop aerial ;in order. to balance out signals from anylparticular .direction since.y

the circuitmis capable ofgbalancing'put signals fromany ,directiczn by, l the. phase and amplitude adjustments .described-..- .Consequently the. 4loop aerial may :be either 0f-the0nventi0nal type lor l may 10,@` a single wire. loop, disposed irregularly around the` walls of a building or other support.k e. When a loop aerial is employed in.thisvway,.

discrimination between wanted and unwantedv i Signals is afforded.. notoily .byany diierencein theirgirequenies but alsato. .a much .greater .ex-k tentjby any dilerence the. direction` from. which signals reacnthe loop. ItV has been shown that,

there is a possible phase variability amounting posing the two-output potentials and for reducing .l5 v.the output of one. oriother of the valves for exact to phase reversal betweenlthe .potentials .set up;

` across the resistance B., and impedance Z and there is also much .variability in amplitude-When ysignals reach. theraerial from (different directions sincel the loopv isstrongly directional. ,.v, The conableamplicationfactor offers a ready means of cuit illustrated in Figure 6, the values rof the resistance. .Ptv and impedance Z respectively are such-gastolprovide only .partialmatching of amplitude, final balancebeing achieved in a circuit fed by'valves. Ifhegrid of. the valve Vl is con. nectedto one end of theresistance and the gridl of;` the valve/.VZVtogthe.other end of the resistance. g '1`l1e.1value: of, -thejresistance R is made smallcompared with the value of the impedance Z-so` that. the potentialexisting acrossthe resistance is always -smaller than that across the impedance. On` the. anode side of` the valves various-known methods may be adopted for opamplitude matching. The use of valves of variachieving .l balancein the anode circuit.v The valves. may amplify at signal frequency or may 20K. dition's. Qf. matched.- phase'and amplitude requiredV forbalance Vwill accordinglybe upset to a great extent by differences in the directionfrom whichv signals reach the aerial and the discrimination..

afforded `by frequency diierence becomes of secondary importance. vA loopaerial circuitem-` ployedin this Way .dscriminates not only be- -f tween signals of diierent frequencies butalso between signals of the vsamefrequency provided they reach the aerial from different directions.

Dilerences of *polarization of the waves have an'i effect similar to diierences of direction. In cases be.. arranged as frequency. changers in a superheterodyne receiver. The circuit shown in Fig-` ure 6` will normally include inthe grid connections the .-.usualgrid condensers, grid leaks and cathode connections tothe ground which are .merely omitted forxsimple illustration.

In cases where the amplitude s balance' .is eiectedfby valvesasprevouslydescribed, the arrangement hasseveralA advantages over the rst v described.circuitsuy The values of the resistance where asmall rotatable loop islemployed instead of the fixed loop referred to above, the normal operation of turning Ythe loop Vto pick up mini-g mum interferenceilnay be employed as an addi-1 tional or alternativeadju'stment, since turning the loop will vary the phase. and amplitude .of`

thev potentials set up across R and Z.v

The impedance. in `theA circuits described lVrnay'V alternatively, see`, lig..5, comprise an. oscillatory.A

circuit tuned to the frequency of theunWanted signals,v .the inductance of this second oscillatory circuit being arranged as a loopaerial or coupled or'directly connectedtoya separate open aerial with or without intermediate valve amplifying stages. The secondv loop or open aerialis spaced f from thel loop or open aerial providing `the inputv to the oscillatorycircuit viirst describedas` yin--g` cluding a resistance andbeing earthed through' an impedance. Wheretwo aerials are thusemf ployed, discrimination may be eiected when-the Y L ratio ol wanted to unwanted signalsis greater i at one aerial locationthan at thev ether. Un-

wanted signals mayberbalanced out at `the tap-V ping point onV theV resistance by vfulfilling the previously described requirements for matching the phase .andi amplitude of thepotentialsfdem veloped across theresistanceand the impedance (inthis case thersecond oscillatory circuit). It

is recognised that exact phase matching is not always possible with inputs from two aerials but somedegree of discrimination1 between wanted and unwanted signalsor interference maynever-v l thelessbe achieved. v

Other specic embodiments4ofthe.invention'r relate to alternative arrangements for matching the amplitude. ofthe potentials to be opposedv in order that unwanted signalsor interference may be balanced out at the, output point Vcon-` nected to the subsequent utilization circuit.

These arrangements apply .either fto aV loop ,cir-i,

cuit or to a circuit having separate inputcon.-

nections.v ..4 i y.

In a modified arrangement ofthe loutputcirand thefimpedancezmay be permanently fixed: without reference toindividual `signal conditions. 'Ihe capacity parallel with theresistance duek tothe. output/.connections is reduced to a 4minivmurnby ireasonof :the direct connection to AtheY grids*` of .the valves: .The value of theresistance vmay..bezlow so thatthe oscillatory circuit is not unduly'2damped, a' particular advantage when thecircuits.has todeal with short wave signals.

Furthermore, l the .arrangement simplifies the phasing operationE when a loop aerial is employed since :balance may vbe lachieved Awhen thepotentialsacross thev resistance and theimpedance are eitherin-phase orphase opposition; When they .are -inpnasefthepotential at the lend of the Vresistance, where itisfconnectedto the condenser,

will` fbe somewhat `greater than. the Vpotential vat the other end of the resistance at its point of connection with the impedance, the potentials being of the same sign;` Whentheyare in phase opposition the potential at the condenser end oi' theflres'istance will besomewhat less than at the other 'end of the resistance at the point "ofv con- `nection'withthe impedance, though 'still of` thev s saine'signn Consequently the switch, previously l Theion-ly phasing adjustment necessary is elected ranged as limit'ers so that their anode outputs are constant for any amplitude input above fa' xedvalue.` fThelimi'ted outputs are matchedin amplitude and"Y opposed as .previously described. The potentialsacross4 Rand Z due tov unwanted signals alrejbroughtfinto phase by the tuning adjustmentof` the oscillatory'circuit and no signals are laccordingly. passed on from the opposed anodecirciviitsgofl .the valves.-V The wanted signals which require a diiferent phase setting of the oscillatory circuit tuning will not be balanced out in the opposed 'anode circuits of the valves since they are out of phase and the wanted signals are `accordingly passed on to the subsequent receiving circuits.` In practice it may be desirable for the valvesVfI` and V2` to be the rst stages of two separate amplifier channels, the opposed limiter Vvalves beingA arranged'at a subsequent stage.

Known limiter arrangements other than valves `arealso applicable to this form of the invention, .which is particularly applicable to telegraphic or other code signals.

The application of the invention to the reduction of fading effects of wireless signals lies in the fact-that fading is often caused by waves reaching the receiving aerial by different paths, sometimes assisting and sometimes opposing each other. A loop aerial is employed as previously described to discriminate between waves arriving at different angles, one or other of the waves being treated as an unwanted signal and accordingly suppressed. In certain conditions improved reception is obtained by balancing out the ground Wave and utilizing the wave reflected from the upper atmosphere or vice versa. In most cases, however, it is found that the reflected wave is too variable in direction for effective suppression as an unwanted signal and also too variable in strength for use as a wanted signal. Furthermore, in the case of short wave reception, the ground wave may be absent altogether, the only available signal being provided by two or more reected waves.

An arrangement of the circuit for use in these conditions is shown in Figure 7 which comprises a loop aerial circuit as previously described, the only modification being the provision of two sliding contacts on the resistance R which feed the signal potentials to two separate amplifiers at X and XI. The potentials developed across the resistance and the impedance are approximately matched in amplitude and phase (if necessary by including a change-over switch, see Fig. 4, for the loop aerial inductance) and the positions of the sliders are adjusted so that each of them balances out waves from one direction whilst permitting waves from another direction to deliver a potential to its succeeding amplier. Such balance can be only approximate because the one phase setting of the tuning control has to serve for both directions and the amplitude matching involves an average position for the sliders to meet conditions liable to continuous variation. Nevertheless, since balance at one position involves unbalance at the other position, the combined effect of a number of waves is not the same in both channels at the same instant. By the application of known. electrical or mechanical methods the final output of the system may be selected from whichever of the two receiver channels offers the better signal or the outputs of the two channels may be combined to assist each other. As an alternative to the use of a simple loop aerial, the input arrangement involving two spaced aerials as previously described is particularly suited to the anti-fading form of the invention.

The alternative output arrangement in which amplitude matching is achieved by means of valves may also be adapted to the anti-fading form of the device. In this case the valves are duplicated, one pair of opposed valves feeding one amplifier channel and the other pair feeding the other amplifier channel. One pair of valves maybalance out waves from one direction land the other pair of valves may balance out waves from another direction, the potentials being accordinglyseparated into two channels as before.

The arrangement is shown in Figure 8 in which the anode outputs of valves VI and V2 are opposed and approximately matched in amplitude,- any resultant signal potential being passed on to an amplifier. The anode` outputs of valves V3 and V4 aresimilarly opposed and approximately matched in amplitude, the resultant signal from this pair of valves being passed on to a `second amplifier. `The circuit shown in Figure 8 will normally include in the grid connections the usual grid condensers and grid leaks which are merely omitted for simple illustration. Where the valves employed are of the variable amplification type, a convenient method of achieving the required average setting for amplitude matching is to apply to a valve in one pair a bias potential which is slightly different from that applied to the valve occupying the equivalent position in the other pair. With this arrangement there remains one valve in each pair which may carry the same bias and if desired economy can be effected by replacing these two valves by one valve whose anode output is shared between the two channels.

The application of the invention to produce a circuit with a resonance curve which is flat over a band of frequencies or which may rise on each side of a carrier frequency may be illustrated by the basic arrangement shown in Figure l. The input potentials are applied to the oscillatory circuit comprising the inductance L and condenser C and the potentials developed across the resistance R and impedance Z are matched for phase and amplitude as before so that they may be balanced out at the point X if desired, In this application of the invention, however, complete balance will not normally be required. A carrier frequency may be partly balanced out and due to the fact that the sidebands are represented by different frequencies they will not balance out to the same extent. The output may therefore have a rising response curve as the frequency of the sidebands departs from the carrier frequency, or this rising response may be used to offset an opposite effect elsewhere in the system to produce an overall response curve which is substantially flat over a band of frequencies. The alternative input and output arrangements relating to the suppression of signals by frequency discrimination may also be applied to this application of the invention. The operations are essentially the same and differ only in degree since in the present case partial suppression only is required.

In the claim the tuning circuit and the impedance connection, for convenience, are defined as follows, to-wit, a closed circuit and a vertical circuit, respectively. It will be noted that the impedance, per se, is not a circuit but is a part of a circuit, which is termed vertical and which shares a common path with the tuning circuit.

The terminology, therefore, of closed and vertical circuits appears most appropriate to indicate one complete side of the entire system whether of input or output type; since in most instances, the disclosed portion of the entire system is interchangeable.

What I claim is:

A selective signal system including a closed tunable circuit and a second circuit, said cirpedance having at one end a ground connection F10 and its sole other end connected to the junctionconnection above, at least an appreciable 'portion of said pure resistanceand said entire'irnpedance being included inv thesecon'd circuit, the de-` V rv'eloped potential of which vis'in phase withfthe `iux` in spacev of the incoming wave signal,Y the impedance by reason of its connection tothe ljun'ctioniearthingnV 'unsymmetrcally the 'flrst vnainedclosed' circuitf andrmeans connected vto 'both' circuitsV for potential delivery, the phases andamplitudesofithe speciedv potentials ef- 'fecting cancellation`VA of unwanted potentials at theipontof delivery of wanted potentials, all points of 'wanted' potential delivery being` remote f frrn "the 'inductanc'e-condenser connection in vsaid closed circuit, said system discriminating between Awanted and unwanted potentials. 'f

' y l 'GEORGEl-IODGESH 

